For the past 2-3 decades, the production of welded metal bellows has been a very labor-intensive task. Typically, a number of stamped circular metal diaphragms about 0.004xe2x80x3 thick are loaded in pairs into a clamping mechanism, and the clamp closed The operator then welds the inside diameter (ID) of the diaphragms using conventional welding methods such as a tungsten-inert-gas (TIG) welding torch, plasma, electron beam, NMG, gas, and/or GTAW. After a number of these welded pairs of diaphragms are made, for example via TIG, they are loaded onto an arbor between copper heat dispersing rings, and end plates clamped against the assembly to hold the metal diaphragms in tight contact with their neighboring part. The assembly is then mounted in a lathe, and rotated at a slow constant speed under a second TIG welding torch. The operator then adjusts the position of the welding torch so as to bring the welding tip in close proximity to the first seam of two of the diaphragms. An arc is struck between the torch tip and the seam, and the operator carefully follows, utilizing a microscope, any deviations in the seam path with the torch tip. By following the deviations in the seam path, the operator can maintain an arc that is fairly well centered on the diaphragm seam. After completing a weld, the TIG torch is moved to the next seam location and the procedure is repeated. This has been the standard method of producing welded metal bellows since their inception in the last 2-3 decades.
This method can be very tiring for the operator, who may be required to weld many convolutions to produce a single bellows capsule. Welding speeds using this method depends very much on operator skill level, with the top speed for an experienced welder being limited by the manual adjustments that have to be made during the weld sequence. Typical welding speeds thus vary from 10xe2x80x3 to 12xe2x80x3 per minute for a typical 0.008xe2x80x3 thick diaphragm pair. Accordingly, the entire welding sequence from first weld to last weld may require a skilled operator an hour or more, and an unskilled or semi-skilled operator considerably more time. Most, if not all, of the welding is done while the operator monitors the operation through a microscope. Weld gas such as argon is directed to pour over the weld area during the weld sequence, potentially exposing the operator to the long term effects of mild oxygen starvation. The weld gas is allowed to flow continuously since the weld area should be open for operator access. Due to the small diaphragm dimensions, it is difficult to cost effectively automate this process, while maintaining the consistency, weld integrity, and reliability obtained with skilled welding operators.
There have been attempts by laser manufacturers and welded bellows manufacturers to utilize a laser in this area of processing. A number of bellows manufacturers have been involved in some experiments with laser welding. However, the reliability and repeatability of laser edge-welding methods has not been sufficient, and, therefore, production of laser welded bellows has been limited to prototype and test production quantities.
One major drawback of using a laser beam for this type of welding process is that the laser beam dimensions are typically very small. The laser beams also typically have very high power densities. In fact, power densities as high as several Mwatts/cm2 are far hotter than any TIG torches presently used for this type of work, and can instantly melt most materials put in range of its focus. Consequently, welding fixtures used for laser processing should be made with extreme precision. Deviations in the weld seam can occur because of, for example, machining tolerances. These deviations can cause the weld seam to wander from side to side as the bellows assembly is rotated under the TIG torch. The manual methods of welding bellows diaphragms relies heavily on operator skills to correct for any deviation in the weld seam. It requires the skills of a seasoned operator to produce consistently good welds. Upon completion of the welding, it is common practice to inspect each weld bead. Such inspection is also a painstaking exercise that is currently undertaken manually. An operator utilizes a microscope to examine each weld bead as it leaves the weld station. If any poor or below standard welds are used in any bellows assembly, early failure during exercising will often occur. Any incomplete welds can result in vacuum leaks, and usually the scrapping of the complete bellows.
Another issue associated with laser processing is known in the industry as xe2x80x98spatial mode hoppingxe2x80x99. When a laser is operating, a profile of the energy distribution within the beam may indicate that the beam is hottest in the central area. However, adjustment of the laser resonator mirrors or a change in the pump power to the arclamp can change the spatial distribution, so that the hottest part of the beam may appear to move around within the beam cross-section. In practice, the spatial mode of a high-power laser system frequently changes as the laser power is programmed to increase, or decrease, at the start, or end, of the weld sequence. With a static beam, these variations in power density within the beam cross-section are often reflected in the molten material of the weld pool. Because of the small size of the weld pool, and the motion of the seam away from the molten area, rapid hardening of the molten material occurs upon leaving the weld pool. Thus any asymmetric melt due to variations in power density within the laser beam can be preserved in the weld, resulting in a lopsided appearance of the weld bead. This asymmetry can give rise to unwelded sections, thin or broken weld beads, and/or non-uniform stress characteristics. Often, this results in cracking during test cycling of the finished products, rendering them useless. One way to potentially overcome this problem would be to use a laser with a constant low order mode profile. That is, use a laser which can maintain a closely controlled power distribution profile across the diameter of the laser beam throughout the range of laser output power. However, a constant power distribution across the diameter of the laser beam is difficult, if not impossible, to achieve, and even more difficult to maintain.
As mentioned, deviations in the weld seam can occur, for example, due to machining tolerances. There have been many methods investigated to maintain the precise alignment required between the focused laser beam and the weld seam. Some of these methods involve magnified vision systems, tactile sensors, magnetic and capacitive proximity sensors, and digital signal processing (DSP) techniques. The method disclosed by Chang in U.S. Pat. No. 6,040,550 uses a magnified vision system and computer to locate the center of the weld seam. The laser beam position is then controlled with mirrors in an effort to follow any meandering in the weld seam path. However, this system is vulnerable to vibration, and can quickly lose accuracy of alignment. The Chang system has no compensation for thermal effects such that the system alignment can drift throughout the day with temperature, adversely affecting performance. In fact, many of the prior methods are difficult to design, expensive, and require careful setting up and maintenance. In addition, the prior methods can be prone to problems due to vibration, temperature and/or radio frequency interference (RFD) from the laser systems.
The subject invention relates to a method and apparatus for welding. The subject invention can be used in the manufacturing of welded metal bellows. The subject method can allow higher manufacturing throughput of finished product, with consistently better quality and less operator attention than the present methods. In addition, the subject technique can reduce the volume of weld shielding gas which is consumed during the welding process, and Java reduce contamination by automation of parts handling equipment. The higher production methods, with reduced operating costs and reduction in operator skill level required, can result in cost savings during the manufacture of welded metal bellows.
The subject invention pertains to a method and apparatus for welding. In a specific embodiment, the metal bellows can be welded using heat provided by a laser beam to perform such welding. The subject invention can allow faster and more reliable production of edge-welded metal bellows using automatic methods. In a specific embodiment, the subject invention can be utilized for the welding of metal bellows diaphragms to form convolution pairs, and bellows capsules. The subject invention also relates to a machine designed for welding the outside seam of two diaphragms, an outside diameter (OD) welding system, and to a machine designed for welding the inside seams, an inside diameter (ID) welding system.
The methods and apparatus of the subject invention can deliver a laser beam so as to get a deep weld penetration and a symmetrical weld bead. In addition, the subject invention can accomplish accurate welding without the necessity for an elaborate seam tracking device. In a specific embodiment, the subject invention can incorporate weld rings which can reflect at least a portion of the laser energy which is off-axis back onto the side of the seam. Reflecting this off-axis laser energy back onto the side of the seam can produce a more efficiently melted pool, and can enable deeper weld penetration. The subject method can introduce the laser energy all around the weld seam, which can be more efficient than introducing the laser energy only on the surface in a direct line facing the laser beam. The subject invention can also incorporate a mechanism for automatic loading and unloading of bellow diaphragms into the weld clamp area reducing human errors and contamination problems due to handling.
In operation, the inside seams are preferably welded first, as is typically done with conventional welding methods. Referring to FIG. 2, a system for accomplishing this task is shown. A female diaphragm 42 can be placed in a rotating clamping fixture, and supported on the inside edge by a lower, fixed, ID clamp face. A male diaphragm 41 can then be inverted and placed on top of the female diaphragm and the upper, movable, clamp brought down under preset air pressure, the air pressure being sufficient to hold them firmly together. The clamp mechanism can then be made to rotate at a steady speed by a DC electric motor 34 under computer control. In this design, the laser beam can be split into two beams by a 50:50 beamsplitter 23 at the appropriate wavelength. The split beams can then be redirected by a prism 24 and scanning mirrors 38 and 39 through the focusing lenses 40 and 46 onto the weld seam. The two mirrors 38 and 39 can be fixed frequency scanners also referred as resonant scanners, and be used to position the beam onto a weld seam. In the static mode, i.e., with the power to the scanners off, the focused laser beams are directed to the upper and lower edges of the weld seam, as shown in FIG. 6. During the weld process, scanning mirrors 38 and 39 can be made to vibrate at a high rate of speed in order to cause the focused laser spot to move across the weld seam. In a specific embodiment, the focused laser spot can move across the weld seam at the focal plane. In a specific embodiment, the focused laser spot can scan back and forth perpendicular to the weld seam.
Referring to FIG. 3, a schematic illustration of how a laser beam can be scanned back and forth is shown. A static beam position is shown as a solid line 21, striking the front reflecting surface of the scanning mirror 38. The reflection coming off passes through focusing lens 40, preferably at near normal incidence, and focuses onto focal plane 72. As mirror 38 oscillates over a half cycle, the deflection 38a of the mirror through angle xcex81 causes the reflected laser beam to be deflected through the angle xcex82, where xcex82=2xcex81. The angular path through the lens repositions the focal point of the beam 71a. On the other half cycle, the beam shifts over to an approximately equal distance on the other side of static point 71. The amount of deflection on the mirror can be small, and can cause twice the angular deflection of the laser beam through optical leverage. Thus, depending on the frequency of oscillation, the laser focused spot can appear as a uniform line heat source rather than as a point heat source. Accordingly, the location of the heating can be controlled without, for example, requiring a non-shifting low-order spatial mode in the laser beam. Furthermore, the subject scanning laser beam can reduce the need for an expensive seam tracking system.
Both the inside diameter (ID) welding machine and the outside diameter (OD) welding machine laser beam optical delivery systems can work on the same principle. In the case of the OD machine, the laser beam can be preferably introduced to the weld seam at right angles to the seam, as depicted in FIG. 5, whereas in the ID machine, the laser beam can be introduced at an angle, for example, 45 degrees, to the seam as shown in FIG. 6. If desired, the laser beam can be introduced at other angles for the OD and ID machines. For example, the beam can be introduced at about a 45 degree angle for the OD machine. In the case of the beam delivery perpendicular to the weld seam, other techniques can be used to redirect the beam back onto the weldjoint, for example, where the beam is scanning past the edges of the seam, beveled weld rings can be designed to reflect the overshoot bean back onto the weld joint. In a specific embodiment, the ID and OD welding can be accomplished by a single welding machine.